In the field of electrical machining the machining fluid has been recognized to be extremely important. In electrical discharge machining (EDM), for example, two sorts of the machining fluid have been currently in use. Thus, in the ram-type EDM used for drilling or cavity-sinking, a hydrocarbon liquid (oil) such as kerosene (paraffin) or transformer oil has been commonly used. In the wire-cut or traveling-wire EDM, it has been common to use a water liquid and, especially, distilled water.
Kerosene has advantageously been used in the ram-type EDM not only for its non-corrosiveness but for its superiority in removal rate capability to water in general and its uniform usefulness over the extensive ranges of machining setting conditions, viz. from a finish range which enables a finer surface finish to be achieved at the expense of removal rate to a roughing range which enables a higher removal rate to be achieved at the expense of surface finish. Furthermore, kerosene is available at a relatively reasonable price and has a relatively long service life. In addition, it can be processed relatively readily for disposal and gives rise to no particular pollution problem when treated properly, although kerosene, like other hydrocarbon liquids, tends to roughen the operator's skin.
Kerosene and other hydrocarbons such as transformer oil are, however, dangerously disadvantageous in that they are inflammable. In the course of a machining operation, when the surface of the hydrocarbon machining liquid in which the tool electrode and the workpiece are spacedly juxtaposed to define a machining gap happens to drop so that the separation of the gap from air (oxygen) is broken, the hydrocarbon liquid in the gap can be ignited by the electrical discharges, thus causing a fire. Thus, extreme care must be exercised by the operator throughout the machining operation vis a vis the inflammability of the machining liquid and a fire extinguisher must be provided. It must also be noted that kerosene for this reason cannot be used in those traveling-wire EDM operations in which the machining gap is defined in air.
On the other hand, water is advantageous in that it is entirely free from the danger of fire, is available at a very reasonable cost, presents no hazard to the operator's skin and is readily processable for disposal or recycling. Water is, however, highly disadvantageous in that it is inferior in removal rate capabilities, especially in medium and roughing ranges. When the existing water liquid is used in these ranges, machining becomes difficult or cannot be performed at a reasonable removal rate. In addition, water can in general not be used in a "no wear" or "low wear" machining mode. For these reasons, it is the state of the art that water has found its use almost exclusively in the traveling-wire EDM, in spite of its definite disadvantages as mentioned above.
In discussing the conventional use of electrical machining fluids, it should also be mentioned that the use of an aqueous solution has been generally known. Thus, in electrochemical machining (ECM) and electrochemical-discharge machining (ECDM), it is commonly required to use an aqueous solution of an electrolyte since these processes commonly entail at least in part the electrolytic dissolution of material from the workpiece. In the field of EDM, there has been a proposal to use an aqueous solution of a certain organic substance. For example, U.S. Pat. No. 3,168,638 to M. J. RIDDLES and U.S. Pat. No. 3,334,210 disclose aqueous solutions containing polyethylene glycol which are claimed to allow an increased "metal removal efficiency" to be achieved. Further, a conventional rust inhibitor may be added to each of these machining fluids. Unfortunately, however, these proposed EDM fluids have not been put into practical use. First of all, these fluids are relatively costly and not available at a reasonable price. Secondly, the machining results, all in terms of removal rate, relative electrode wear and surface finish, obtainable with these fluids are still much inferior to those achieved with a hydrocarbon such as kerosene. Evidently, this is due to the fact that discharge efficiency is still very deficient compared with that achieved with the hydrocarbon liquid. We have recognized that the discharge decomposition products from these fluids in the machining gap are, in major part, a hydroxide and carbonyl base which tend to substantially reduce the resistance of the machining gap so that the discharge efficiency of the machining process remains unsatisfactory.
It has now been discovered that a water-based machining liquid, when it contains an additional component in the form of a surface active agent such as a silicone oil, suffers less drop in its resistance and permits electrical machining current, e.g. in the form of successive electrical discharges, to proceed with increased efficiency. Sometimes it is also desirable that the water-based liquid include a further additional amount of a liquid hydrocarbon to improve electrical machining performance and discharge stability. When the liquid hydrocarbon is added together with a surface active agent to the water, it can be present in a highly uniform emulsified state in the solution.
In an electrical machining system utilizing such an improved, highly favorable solution, however, it has been found that problems arise when an attempt to recycle one or more components thereof is made. While water and a liquid hydrocarbon are relatively cheap, the surface active agent is of considerable expense. The effluent drained from the electrical machining zone contains machining products, e.g. chips and/or sludge, entrapped in suspension in residual amounts of deionized water and surface active agents. It has been found that machining chips contain extremely fine metal particles, e.g. of a particle size of micron order, which are firmly trapped in the solution. It has thus been found to be extremely difficult to recover these machining products from the machining liquid effluent which consists basically of a highly homogeneous solution of the surface active agent with a deionized water and to recover the surface active agent from the water-based effluent containing these machining products. The difficulty is increased further when the liquid contains a hydrocarbon. Conventional filtration and separation techniques hitherto employed in the art are of little help in overcoming these difficulties.